1. Field of the Invention
This invention relates to a pressure-sensitive recording material. More particularly, the invention relates to a pressure-sensitive recording material which is applied with microcapsules containing a solution of dyestuff or coloring agent in a heavy fraction of distillation. The heavy fraction is obtained by treating a raw material fraction in the presence of an acid catalyst and the above raw material fraction is obtained by the thermal cracking of petroleum hydrocarbons.
2. Description of the Prior Art
Several kinds of pressure-sensitive recording materials are hitherto well known. For example, a sheet of paper is applied with microcapsules containing a solution of a colorless dyestuff and another sheet of paper is applied with clay or polymeric material which produces a color by reacting with the dyestuff. When the recording materials are used, the treated surfaces of the above sheets are put together face to face and local pressure is applied to the paired papers by hand writing or typewriting, thereby obtaining desired duplicate impressions.
The recording mechanism in the pressure-sensitive recording material of the type described above is such that the microcapsules are ruptured by the pressure of hand writing or the impacts of typewriting to release the coloring dyestuff solution from the microcapsules. The dyestuff solution contains a coloring agent as an electron donor dyestuff and comes into contact with the clay or polymeric material as an electron acceptor on the opposing surface of the other sheet of paper, thereby producing a color.
In another type of known recording material, the microcapsule layer is applied on one side of a sheet of paper as an inner layer and the clay or polymeric material is then applied an an outer layer over the microcapsule layer. Similarly, when it is used, the microcapsules on this recording material are ruptured by the pressure of hand writing or typewriting and the coloring solution containing a dyestuff is released from the microcapsules, the coloring solution coming into contact with the clay or the polymeric material in the outer layer, thereby producing a color.
The coloring solution that is used for these recording materials is a solution of a colorless dyestuff of an electron donor in one or more kinds of hydrophobic solvents. This hydrophobic solvent is required to have the following properties.
That is, the solvents must have the properties of: innoxiousness, no disagreeable odor, colorless or quite light color, non-volatility, good dissolving power with dyestuffs and good stability when a dyestuff is dissolved. It is further required that, in the preparation of microcapsules, a very fine and stable disperesion of the solution can be produced; microcapsule membranes can be formed around the fine particles of the dispersion; the obtained microcapsules have good storage stability; the membranes of microcapsules are uniform and of desired thickness; the solvent does not inhibit the color-producing reaction between a dyestuff and clay or a polymeric material with a high reaction rate; the solvent dissolved the polymeric material to cause close contact with the dyestuff when the base paper is coated with the polymeric material; the duplicate impression is clear without runs; and that the duplicate impression can be maintained as it stands for a long period of time without any change.
In the prior art, polychlorinated biphenyl was widely used as the solvent for producing the microcapsules of these pressure-sensitive recording material. The polychlorinated biphenyl has indeed several excellent properties as the solvent of this kind. It has, however, a serious disadvantage in that it is quite toxic to health and its accumulation in the human body can cause several disorders. Therefore, when the recording material having a microcapsule layer containing the polychlorinated biphenyl is handled in the production process and used in recording, it gives serious problems. Accordingly, it is required that the solvent dissolves dyestuffs well, has excellent properties for the microcapsule preparation and is non-toxic.
Among solvents which meet such requirements, the already proposed desirable ones are non-condensed bicyclic aromatic hydrocarbons, that is, diaryl alkanes, which are disclosed in U.S. Pat. No 3,836,383 (Kiritani et al.), British Pat. No. 1,406,107 (Monsanto Co.) and U.S. Pat. No. 3,936,566 (Sato and Shimizu, both are the inventors of the present invention, and another person).
In the above prior art references, the methods for preparing the solvents are disclosed, in which styrenes or benzyl chloride is caused to react with alkylbenzenes, or formaldehyde is caused to react with alkylbenzenes. In these preparation methods, any of the styrenes, benzyl chloride and formaldehyde are expensive materials which must be produced by synthesis. Further, the alkylbenzenes are also expensive since they are also produced by synthesis or aromatic group extraction. Therefore, these methods are not satisfactory for providing inexpensive solvents.